The present invention generally relates to systems for identifying pulses of light. More particularly, the invention relates to systems for sensing tracking holes in tapes, such as data tape streamers.
In conventional tape/media hole or end-of-media sensor systems, light emitting diodes (LEDs) or incandescent lamps are used as light sources. A tape generally is contained within a cassette and light beams generated by these light sources are directed onto the tape either directly or via a mirror. Discrete phototransistors are normally used to sense light penetrating through holes in the tape. The phototransistors are used both as light sensors and signal preamplifiers.
The discrete phototransistors generally are placed in nominal positions where the light beams from the light sources are expected to strike as the holes in the tape pass thereby. Thus, it is necessary to properly position the phototransistors to detect the tape holes.
The difficulties that arise during the production of such systems are well-known. These systems typically require some form of preamplifier gain adjustment to establish or set the signal level in front of a fixed threshold amplifier or comparator. This is normally provided by means of a potentiometer. However, designing for a fixed preamplifier gain is difficult and costly to implement in volume production because the spreads in optical and electrical DC transfer ratios are very large.
Further, the spread in light output is nearly always greater than 4:1 for the light sources or emitters, typically being 6:1 when the maximum and minimum values are specified for standard parts.
In addition to the foregoing, the emitted light will also vary with temperature, for example, typically by a ratio of 1.3:1. The gain spread for phototransistors is typically specified to be 2:1 at a fixed temperature, often more for standard parts.
Yet further, the dark currents of phototransistors vary greatly with temperature and this can easily cause a drift on the order of 100 millivolts in the DC output for worst case samples of phototransistors with large collector loads.
The calculations required to compensate for the foregoing variations are further complicated by the mechanical tolerances of the optical path in these systems including the tolerances in the placement of the light sources and the light sensors. Low cost discrete components tend to vary greatly in operation and there is a need to include large margins for noise and reasonable dynamic range. Accordingly, these sensor systems in a typical tape streamer are constructed for a 12-volt operation.
The presently available integrated sensor systems are sensitive and pick-up stray light which is often difficult to eliminate. The threshold levels thus specified for the systems are specified for the constant light input and hence there are small margins only provided for noise pick-up. Therefore, relatively large currents are needed in some applications to try the light sources to obtain good signal-to-noise ratios.
In these systems, the threshold level typically varies with temperature from +/-10% to +/-20%, depending on the operating temperature range. To reduce the stray light pick-up problems and to increase the efficiency, infrared light sources and visible light cut-off filters have been used. In some applications requiring even more ambient noise suppression, synchronous demodulators are included on the integrated sensor chip. The sensor systems also include specifications for the ambient light.
In addition to the foregoing, the oscillator and driver for the light emitting diodes used as light sources are included in the synchronous receiver which may be disadvantageous in applications where the light source and light detector are physically separated.
Another problem that arises with respect to synchronous detectors is the excessive time delays or phase shifts which may occur for the light emitting diodes. Depending upon the type of emitters used, the phase shift may cause problems above, for example, 10-20 kH. In one solution to this problem, a carrier regenerator has been built into the receiver to reliably demodulate the signal. For many applications, for example magnetic tape hole detection, a frequency below 10 kH, as is used for synchronous detectors, is too low because the tape can be run at a speed of 120 inches/second.
A typical integrated photosensor is disclosed in a Data Book provided by OPTEK Technology, Inc. bearing publishing dates of 1989 and 1990. Copies of 1-12 and 1-13 of this data book are simultaneously filed herewith and freely incorporated by reference.
A general description of opto-electronic devices, including an integrated device, is provided in an Optoelectronics Data Book provided by Sharp Corporation bearing publishing dates of 1988 and 1989. Pages 24-33 thereof are fully incorporated herein by reference.
A description of one high speed response type OPIC light detector with a built-in signal processing circuit for a light modulation system is provided in a publication entitled "Optoelectronics Data Book - Supplementary Edition, 1990." This publication is also provided by Sharp Corporation. Pages 38 to 46 thereof are fully incorporated herein by reference.